This invention relates to measuring electron beam parameters, and, more particularly, to measuring the length of subpicosecond electron beam bunches.
Recent experiments using electron accelerators have demonstrated the generation of subpicosecond electron bunches. These short electron bunches are the enabling technology for many important radiation sources ranging in wavelength from x-rays to the far-infrared. The tunable femtosecond radiation bunches that are produced will greatly benefit the rapidly growing field of ultrafast phenomena, an area spanning femtosecond excitation and ionization of atoms and molecules, to imaging ultrafast motion of biological molecules. The femtosecond electron bunches can also be used to produce wakefield accelerators, an advanced accelerator concept that could usher mankind into a new era of high-energy physics.
As the generated electron bunch lengths get shorter, it becomes increasingly difficult to measure the bunch length. Unfortunately, diagnostic techniques for electron beam bunches have not kept pace with the technology for producing short electron bunch lengths. The present technique for measuring the length of sub-picosecond electron bunches fall into three main categories: the time-domain methods using a streak camera, the frequency-domain methods using either coherent undulator radiation or coherent synchrotron radiation, and the autocorrelation method using a Michelson interferometer. All these methods involve expensive instruments and sophisticated setups. An ideal diagnostic method for measuring electron beam bunch lengths must be inexpensive, easy to set up, non-intercepting, and capable of measuring a single electron bunch.
Time-domain method (Streak camera): This device uses a photocathode to convert incoming light into electrons that are swept by an applied radio-frequency field. The electrons impinge on a phosphorescent screen and cause the latter to emit light. The width of the electron image on the screen is a measure of the electron bunch length. This technique is a direct measurement of the electron bunch length. However, besides being very expensive, the streak camera method cannot measure bunch lengths shorter than 200 fs. Even with longer bunch lengths, a picosecond temporal resolution is difficult to achieve due to temporal jitter inherent in the streak camera. Furthermore, the method requires a means to convert electrons into visible light that falls within the wavelength response curve of the streak camera photocathode. A commonly used method to convert the electron bunches into light pulses is to impinge the electron beam on a metal screen and observe the so-called optical transition radiation (OTR). Because of the low intensity of OTR light, streak camera measurements often require that the measurement be done over many electron bunches. As the OTR screen also disrupts the electron beam, this is not a non-intercepting diagnostic method.
Frequency-domain method: This is an indirect method based on the frequency spectrum of light emitted when electrons traverse an undulator (a series of alternating magnets) or a circular bend. In the first case, the light emitted is called coherent undulator radiation; in the second, coherent synchrotron radiation. The method assumes a knowledge of the electron bunch shape in order to convert spectral information to bunch length. It also requires a spectrometer to measure the spectra of the coherent radiation. The spectra are often collected by scanning the spectrometer, thus making this a time-averaged measurement (not a single-bunch measurement).
Autocorrelation method: This technique uses a device called a Michelson interferometer to measure the overlap between the light pulses that the electrons produce. The electron bunches impinge on a screen to produce coherent OTR light. The light is split into two beams by a beam splitter and then sent to two perfectly aligned mirrors that return the lights back to the beam splitter where they recombine. One of the mirrors is moved to vary the distance between the two light paths. As its distance varies, the light patterns interfere to produce an interferogram from which the bunch length is reconstructed. This technique is indirect, intercepting (because it uses an OTR screen) and not a single-bunch method.
Recently, a group of researchers in Japan (K. Ishi et al., "Observation of coherent Smith-Purcell radiation from short-bunched electrons," 51 Phys. Rev. E, pg. R5212 (1995)) reported the observation of coherent Smith-Purcell radiation emitted from picosecond electron bunches traveling near the surface of a grating--a surface made out of aluminum on which small grooves are cut. In contrast to the usual incoherent Smith-Purcell radiation, the observed coherent Smith-Purcell radiation depends quadratically on the number of electrons in the bunch. The coherent Smith-Purcell radiation is, thus, many orders of magnitude greater than the incoherent radiation. These workers used the phenomenon to study the possibility of producing mm-wave radiation from short, but not subpicosecond, electron bunches. They detected the coherent Smith-Purcell radiation at large angles using a spectrometer.
The present invention recognizes that the coherent Smith-Purcell radiation observed for picosecond electron bunches can be used as a diagnostic for determining electron bunch lengths in electron beams. Electron beam bunches are made to travel above the surface of a grating. The coherent Smith-Purcell radiation is emitted at large angles with respect to the beam propagation direction. The angular distribution of the emitted radiation depends on the electron bunch length so that electron bunch length is directly deduced from measured angles of the emitted Smith-Purcell radiation.
Accordingly, an object of the present invention is to measure ultrashort electron bunch lengths down to a few femtoseconds.
Another object of the present invention is to measure electron bunch length with very few assumptions or complexities to yield a unique measurement of the bunch length.
One other object of the present invention is to provide a non-invasive, non-intercepting method of electron beam diagnostics.
Yet another object of the present invention is to measure the length of a single bunch of electrons.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.